We have investigated a select group of amorphous aromatic polyimides to determine their potential suitability in fabricating thermally stable optical components, intended for use at high temperatures. Compared to current commercial polymers, these polyimides have much higher glass transition and decomposition temperatures, and also have much lower thermo-optic and thermal expansion coefficients. This combination of characteristics makes them potentially useful in a wide range of commercial and military applications. To better understand the optical properties of these polyimides, we have investigated correlations between several aspects of their chemical structures and their thermo-optical properties. Our study consisted of synthesizing variations of these polyimides, which incorporated targeted structural modifications, and then correlating these structures with trends observed in their optical properties. The modifications studied included the substitution of a variety of side groups that affect the low-frequency vibrational modes, the substitution of structural isomers with distinct symmetries, and the introduction of functional groups that alter the monomer-level anisotropy. Furthermore, we also investigated copolymerization in compatible pairs of monomers as a means of fine-tuning the thermo-optical properties.
Recent interest in quantum dots (QDs) stems from the plethora of potential applications that arises from their tunable absorption and emission profiles, high absorption cross sections, resistance to photobleaching, functionalizable surfaces, and physical robustness. The emergent use of QDs in biological imaging exploits these and other intrinsic properties. For example, quantum confined Stark effect (QCSE), which describes changes in the photoluminescence (PL) of QDs driven by the application of an electric field, provides an inherent means of detecting changes in electric fields by monitoring QD emission and thus points to a ready mean of imaging membrane potential (and action potentials) in electrically active cells. Here we examine the changing PL of various QDs subjected to electric fields comparable to those found across a cellular membrane. By pairing static and timeresolved PL measurements, we attempt to understand the mechanism driving electric-field-induced PL quenching and ultimately conclude that ionization plays a substantial role in initiating PL changes in systems where QCSE has traditionally been credited. Expanding on these findings, we explore the rapidity of response of the QD PL to applied electric fields and demonstrate changes amply able to capture the millisecond timescale of cellular action potentials.
We report a study on solar cells using pentacene derivatives with triisopropylsilylethynyl substitution at the
6,13-position and 1,3-dioxolane substitution to the terminal benzenoid rings of pentacene as the electron donor and C60 as the electron acceptor. A significant increase in the open circuit voltage (Voc) was obtained in all the
pentacene-derivative
based cells with the highest Voc as high as 0.90 V, compared to a 0.24 V value for pentacene. The variation in the Voc of
the cells is in qualitative agreement with the larger offset between ionization potential of the electron donor and the
electron affinity of C60. The power conversion efficiency (η) at 100 mW/cm2 of EtTP-5/C60 cells reached 0.74%, which
is comparable to that of a pentacene/C60 cell (0.82%).
We studied hole injection from the conducting polymer blend poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) by optical spectroscopy and characterization of organic light-emitting diodes (OLEDs). Electroabsorption (EA) spectroscopy was used to measure the built-in potential of polyfluorene-based OLEDs with indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) anodes. Although the work function of PEDOT:PSS is 5.1 eV, the inferred anode work function matches the ionization potential of the emitting polymer. We conclude that the Fermi level at the PEDOT:PSS/polyfluorene interface is pinned to the highest-occupied molecular orbital (HOMO) of the emitting polymer, permitting efficient hole injection. To test this hypothesis, we fabricated OLEDs using the archetypical molecular semiconductor, tris(8-hydroxyquinoline) aluminum (III) (Alq3). Although the anticipated hole injection barrier is 0.7 eV, OLEDs with Alq3 deposited onto PEDOT:PSS operate at a lower bias and higher power efficiency than OLEDs with a hole transport layer. The quantum efficiency of single layer Alq3 and rubrene-doped Alq3 devices is equal to that of multi-layer devices, showing that EL is not quenched by PEDOT:PSS.
Single heterojunction and multi-heterojunction, small-molecule organic photovoltaic devices (OPVs) have been prepared on Glass/ITO and fully-flexible thermoplastic substrates using pre-patterned, conducting polymer electrodes (~ 450Ω/□). OPVs were fabricated via sequential vacuum vapor deposition of layers of the organic
electron donating/hole transporting material: N,N'-(a-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (a-NPD) and the electron accepting/transporting material C60. Devices built on glass/ITO substrates operated with a maximum, white-light power conversion efficiency (ηpower) of 1.1% (AM1.5, 97 mW/cm2). Analogous devices fabricated on fully-flexible, plastic substrates using conducting polymer transparent electrodes exhibited white-light power conversion efficiencies of ~1%, virtually identical to those fabricated on prepatterned ITO/glass
substrates. The glass/ITO cells were further optimized by including an exciton blocking layer of 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) and their ηpowerS exhibited a 30% increase to 1.3%.
Laser processing techniques, such as laser direct-write (LDW) and laser sintering, have been used to deposit mesoporous nanocrystalline TiO2 (nc-TiO2) films for use in dye-sensitized solar cells. LDW enables the fabrication of conformal structures containing metals, ceramics, polymers and composites on rigid and flexible substrates without the use of masks or additional patterning techniques. The transferred material maintains a porous, high surface area structure that is ideally suited for dye-sensitized solar cells. In this experiment, a pulsed UV laser (355nm) is used to forward transfer a paste of commercial TiO2 nanopowder (P25) onto transparent conducting electrodes on flexible polyethyleneterephthalate (PET) and rigid glass substrates. For the cells based on flexible PET substrates, the transferred TiO2 layers were sintered using an in-situ laser to improve electron paths without damaging PET substrates. In this paper, we demonstrate the use of laser processing techniques to produce nc-TiO2 films (~10 μm thickness) on glass for use in dye-sensitized solar cells (Voc = 690 mV, Jsc = 8.7 mA/cm2, ff = 0.67, η = 4.0 % at 100 mW/cm2).
This work was supported by the Office of Naval Research.
Investigations into the lithium fluoride and cesium fluoride/tris(quinolin-8-olato) aluminum (LiF-Alq3 or CsF-Alq3) Lewis acid-Lewis base pairs have been undertaken using hybrid density functional calculations. The results of the calculations clearly show that there is a strong interaction that occurs between the metal fluoride and both the facial and the meridional isomers of Alq3. This strong interaction has the effect of modifying the electronic structure of Alq3 and can be thought of as the first step in the formation of the highly effective Alq3/Metal Halide/Al electron injection contact.
Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on glass and on flexible polyethylene teraphthalate (PET) substrates. The structural, electrical and optical properties of these films were investigated as a function of substrate deposition temperature and background gas pressure. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 1.2 J/cm2. Films were deposited at substrate temperatures ranging from 25 degree(s)C to 150 degree(s)C in oxygen pressures ranging from 10 to 60 mTorr. ITO films (280 nm thick), deposited by PLD on PET at 25 degree(s)C and 45 mTorr of oxygen, exhibit a low electrical sheet resistance (20 - 25 (Omega) /sq.) and high transparency (approximately 85%) in the visible range (400-700 nm). We have also used the ITO thin films deposited on both glass and PET substrates by PLD as the anode contact in organic light emitting devices (OLEDs) and measured the device performance. The external quantum efficiency measured at a current density of 250 A/m2 for the device on PET was approximately 0.9%, which is higher than that (approximately 0.5%) for the device on glass. The reduction in the driving voltage and high external quantum efficiency made an ITO coated PET substrate very promising for future large scale OLED application.
Transparent and highly conducting zirconium-doped indium oxide (ZIO) thin films have been grown by pulsed laser deposition (PLD) on glass substrates without a post- deposition anneal. The structural, electrical and optical properties of these films were investigated as a function of film composition and substrate deposition temperature. Films were deposited using a KrF excimer laser (248 nm, 30 ns FWHM) at a fluence of 1 J/cm2 at growth temperatures ranging from 20 degrees Celsius to 400 degrees Celsius in oxygen pressure ranging from 1 mTorr to 25 mTorr. The films (approximately 2000 angstrom thick) deposited at 200 degrees Celsius in 25 mTorr of oxygen show electrical resistivities as low as 2.7 X 10-4 (Omega) -cm, the average visible transmittance of 89%, the refractive index of 1.99 and optical band gap of 4.1 eV. These ZIO films were used as a transparent anode contact in organic light emitting diodes (OLEDs) and the device performance was studied. The external quantum efficiency measured at 100 A/m2 for the [ZIO/TPD/Alq3/MgAg] diodes was about 0.9%. Low driving voltage and high light emission were observed for the OLEDs with the ZIO anode.
The absolute photoluminescence quantum yields ((Phi) PL) of three end-capped oligothiophene derivatives dispersed in N,N'-((alpha) -naphthyl)-N,N'-diphenyl-1,1'-biphenyl ((alpha) -NPD) have been evaluated and the most efficient of the emitters was used as a dopant in molecular organic LEDs. Composite films of 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-furan (EC5FUR); 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl) thien-2-yl]-oxazole (EC5OXZ) and 2,5-bis [5-(4,5,6,7- tetrahydrobenzo[b]thien-2-yl)thien-2-yl]-1,3,4- oxadiazole (EC5OXD) doped into (alpha) -NPD were found to have (Phi) PL values of 78, 62 and 28%, respectively. MOLED devices were fabricated using an EC5FUR/(alpha) -NPD composite as the emitting layer and the external quantum efficiencies ((eta) EL) of these devices were evaluated. The results of the device characterization show that the inclusion of EC5FUR in the NPD hole transport layer increases the device (eta) EL to 1.45% at a current density of 10 mA/cm2. In addition, the concentration dependence of the (eta) EL on the EC5FUR dopant in certain device structures when considered in conjunction with the current results of ultraviolet photoemission spectroscopic experiments suggests that this dopant species may be acting as both a hole and electron trap in the (alpha) -NPD host.
Transparent conducting indium tin oxide (ITO) thin films were grown by pulsed laser deposition (PLD) on glass substrates. The structural, electrical and optical properties of these films were investigated as a function of film thickness. Films were deposited using a KrF excimer laser at a fluence of 2 J/cm2, at substrate temperature of 300 degrees C and 10 mTorr of oxygen pressure. For ITO films deposited at 300 degrees C in 10 mTorr of oxygen pressure, the resistivity of 2-4 X 10-4 (Omega) - cm was observed and the average transmission in the visible range was about 85-90 percent. The Hall mobility and carrier density for ITO films were observed to be in the range of 24-27 cm2/V-s and 5-9 X 1020 cm-3, respectively. We have used the ITO thin films, deposited by PLD on silica substrates, as the anode contact in organic light emitting devices and studied the effect of ITO film thickness on the device performance. The optimum thickness of the ITO anode for the maximum device efficiency was observed to be about 500-1000 angstrom. The device shoed an external quantum efficiency of about 0.8 percent at 100 A/m2.
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